The present invention relates generally to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device more having a semiconductor substrate using an epitaxial growth film in which a bipolar transistor is formed.
Conventionally, in the entire region of or a limited part of a semiconductor substrate using an epitaxial growth film, a buried layer is often provided below the epitaxial growth film. The buried layer is used to lower the ON resistance of a device formed above the buried layer in the epitaxial film as well as to suppress parasitic elements and to improve the soft error and latch-up tolerance of the device.
The buried layer is typically formed by heavily doping impurity in the entire region or a part of the semiconductor substrate and by annealing the doped semiconductor substrate in a thermal diffusion furnace. When an epitaxial growth film is formed on a semiconductor substrate having a buried layer, the impurity in the buried layer is diffused into the epitaxial growth film by solid phase diffusion and autodoping. Then, further diffusion into the epitaxial growth film is carried out by various thermal processes for forming a device in subsequent steps, which is well known.
FIGS. 3a-3d are sectional view of a semiconductor substrate having a buried layer using an epitaxial growth film according to a conventional method of manufacturing a semiconductor device.
A buried layer 114 of an N conductivity type is formed in a silicon semiconductor substrate 101 FIGS. 3a and 3b, and an epitaxial growth film 113 is formed on the silicon semiconductor substrate 101 (FIG. 3c). Impurities in the buried layer 114 are diffused into the epitaxial growth film 113 by solid phase diffusion and autodoping during the formation of the epitaxial growth film 113. Further, the amount of diffusion of the impurity in the buried layer 114 into the epitaxial growth film 113 is increased by various thermal processes performed during the formation of a device in the surface of the epitaxial growth film 113, and thus, it is not easy to decrease the amount of diffusion. The buried layer 114 finally becomes a buried layer 114B. If the amount of diffusion from the buried layer 114B is large, a collector heavily doped region approaches a base region 108 of a bipolar transistor 106 formed thereon, which leads to deterioration in the withstanding pressure. Therefore, when a transistor with high withstanding pressure is prepared, the formed epitaxial growth film 113 has to be made thicker accordingly.
When a bipolar transistor 106 is formed with a buried layer as a part of a collector region, various thermal processes cause the buried layer to expand upward into the epitaxial layer. This makes the buried layer approach a base region too much, which sometimes makes the withstanding ability of the bipolar transistor lower than a desired value. It has of course, been taken into consideration that thickening of the epitaxial growth film is a viable method of increasing the distance between the buried layer and the base region. However, this makes it difficult to form an isolation region. This is because the isolation region is formed by coupling the buried layer and a well region by means of diffusion from the bottom and from the top, respectively. The larger the epitaxial thickness becomes, the more difficult it becomes to form the isolation region.
It may also be taken into consideration that the smaller the less the expansion of the layer the dose in the epitaxial layer. However, the smaller it becomes, the more likely it becomes to cause variation in the concentration when autodoping is carried out during the time the epitaxial layer is grown, and thus the control is difficult.